The present invention relates to an electrorheological fluid comprising a dispersed phase and a base liquid wherein the dispersion consists of finely divided particles of a metal amino acid salt.
Electrorheological (ER) fluids are composed of a polarizable solid phase dispersed in a dielectric fluid phase. ER fluids are unique in that they have the ability to change their characteristics from liquid-like to solid-like upon application of an external voltage. This change is reversible which means that the liquid-like state returns upon removal of the electric field. Upon application of a voltage, the solid particles form fibril-like networks which bridge the electrode gap. At this point, the material will not behave as a Newtonian fluid, but will exhibit a Bingham plastic behavior. Fluids exhibiting the Bingham plastic effect require application of a particular level of force (yield stress) before the material will flow again.
It is desirable in the ER fluid art to improve the strength of such fluids which thereby permits smaller devices requiring less power drive to be built. The production of an ER fluid with greater strength would also allow devices to be operated at lower voltages, which would have advantages in power supply design, and generally would open up other application areas for the use of ER fluids that are currently beyond the capabilities of existing ER fluids. It is also desirable in an electrorheological fluid to match the density of the solid phase with the density of the fluid phase.
Aluminum based particle systems have been described in the art. For example, Goosens et al., in U.S. Pat. No. 4,702,855, discloses ER fluids based on aluminum silicates in an electrically non-conducting liquid and a suitable dispersing agent. The contribution to the art provided by this patent was an improved electroreactivity, as well as improved stability over a wide temperature range. This was accomplished by the addition of certain polysiloxane dispersants to the ER fluid formulations.
There also have been several ER particle systems which have described that either colloidal aluminum or colloidal zirconia or a mixture thereof could be utilized in the disperse phase of an ER fluid. For example, Hashimoto et al., Japanese Patent Application Laid Open 01304188, discloses an electroviscous fluid which consists of 5 to 50 weight percent of a dispersion of particles of 5 to 1000 microns in diameter and 95 to 50 weight percent of a liquid phase of a nonreactive or modified silicone oil having a 0.90 to 1.30 specific gravity. The particles can be one or a mixture of more than one of colloidal silica, colloidal alumina, colloidal zirconia, or antimony oxide.
Japanese Patent Application Laid Open No. 01172496 teaches an electroviscous fluid obtained by dispersing dielectric particles into an oily medium high in electrical insulation, the dielectric particles comprise hollow bodies into which the oily medium will not permeate. Examples of the dielectric particles are the metallic oxides of silica, alumina, silica-alumina, spinel, zirconia, and titanium oxide or vanadium oxide; metals such as aluminum, silicon, nickel or copper; ferroelectric substances such as calcium titanate or strontium titanate; or of a synthetic high polymer such as polyvinylidene fluoride, polyamide or an ion exchange resin. The fluid is disclosed as having long-term stability.
Japanese Patent Application Laid Open No. 03166295 teaches an electroviscous fluid having improved dispersibility comprising dielectric particles dispersed in an electrically insulating liquid which has main particles having a grain size of 3-100 microns and contains finer particles having an average grain size of 0.3 micron to 20% of the average grain size of the main particles. Available materials for the finer particles include polyamides, MgO, Zr oxide, silica, alumina, Ti oxide, and Si nitride. Available materials for main dielectric particles include starch, cellulose, casein, ion exchange resins, silica, alumina, silica-alumina, Al.sub.2,(OH).sub.3, Zn(OH).sub.2, mica, and lithium and potassium tartrate.
Japanese Patent Application Laid Open No. 03200897 discloses a new fluid composition which consists of one or a mixture of inorganic ion-exchanged materials comprising hydroxides of polyvalent metals, acidic salts of polyvalent metals, and potassium titanates dispersed in an electrically insulating dipersion medium. Preferred polyvalent metal hydroxides include titanium, zirconium, and magnesium hydroxide. Acidic salts of polyvalent metals include zirconium phosphate and titanium arsenate.
Other particle systems which have been described in the ER fluid art recently are silicone amine sulfate particles dispersed in polydimethylsiloxane fluid as described in U.S. Pat. No. 4,994,198, and lithium-polymethylmethacrylate particles dispersed in a chlorinated paraffin base fluid as described in Great Britain Unexamined Application No. 1570234.
However none of the references described hereinabove teach a hydrolyzable metal amino acid salt which produces an electrorheological fluid having unexpectedly high yield stress values while retaining good dispersion stability in compatible base liquids.